1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. More particularly, the present invention relates to a method of manufacturing an aluminum gate electrode of a thin film transistor.
2. Description of the Related Art
Typically, an amorphous silicon thin film transistor is used as a switch device for an active matrix liquid crystal display (LCD) since the advantages of the amorphous silicon include low temperature deposition, low cost, less leakage current and large area production. However, the quality of the amorphous silicon thin film transistor becomes poorer because the aluminum gate electrode of the amorphous silicon thin film transistor protuberates when the temperature is raised, to manifest a hillock effect.
Since the difference between the thermal expansion coefficient of the aluminum electrode and that of insulating layer of the substrate is very large, the stress of the aluminum electrode is different from that of the insulating layer of the substrate when the temperature is raised. Moreover, because of the low melting point of aluminum, the vacancy of the aluminum electrode rapidly diffuses to release the stress in the aluminum electrode as the temperature rises. Therefore, the aluminum electrode protuberates when the temperature is high.
Commonly, the ways to constrict the hillock effect includes the following methods:
1. A metal layer such as a tungsten layer or a titanium layer deposited on the aluminum electrode is used to inhibit the hillock effect ill a direction vertical to the substrate surface. However, the aluminum electrode then laterally protuberates. Because of the laterally protuberating aluminum, the aluminum electrode electrically couples to the adjacent devices when the size of the devices and the spaces between devices become smaller. PA1 2. Several elements are added into the aluminum electrode, in an amount larger than the maximum solid solubility of the aluminum to produce additional precipitation at the grain boundary. Since the education can avoid the vacancy diffusion, the hillock. effect can be avoided. But this method cannot effectively inhibit the hillock effect. Incidentally, the education easily diffuses into the gate dielectric layer when the temperature lowers rapidly Therefore, the quality of the gate dielectric layer worsens. PA1 3. Silicon elements are added into the aluminum electrode and then a titanium layer is formed on the aluminum electrode. Although the hillock effect can be avoided, the operation rate of the devices becomes slow since the resistance of the aluminum electrode having the dopants of the silicon elements is very large.